Harry Rosen, UNIDO pump expert and MD of TAS Online, uses pump and system curves from typical pumping systems to explain scenarios where variable speed drives (VSDs) are an excellent choice for realising substantial energy saving and, most importantly, those pump systems where a VSD will not improve energy efficiency or pumping performance.
Installing a variable speed drive (VSD) will not always improve the energy efficiency or performance of a pumping system. In fact, in some systems, pump performance will be compromised and energy use will increase when the pump speed is reduced.
Variable speed drives for pumping systems are not a new concept. They have long been used to control the flow in pumping system, as an alternative to stop-start pumping, throttling using a flow restricting control valve, using a bypass valve to recirculate excess flow back to the inlet or using multiple pumps in parallel. Other strategies like trimming the pump impeller or changing the V-belt pulley size to reduce the input drive speed from the motor, are of a more permanent nature and cannot react to real time changes in the system.
Superficially, using some sort of variable speed drive to reduce the flow to match demand seems to be a more efficient way of managing flow requirement from a pump. But this depends on the pumping system, most notably, the balance between the portion of system pressure that is due to friction losses and that portion needed to overcome the static head.
The affinity laws and pump system curves
According to the affinity laws for centrifugal system: reducing the pump speed by 20% will reduce the flow by 20%, but will reduce the input power by 50%. This applies to pumps and fans, where the power absorbed by a centrifugal pump varies with the cube of the pump speed. That means that small decreases in speed can significantly reduce energy use, which implies a great opportunity for installing variable speed drives (VSDs) in pumping applications.
But the fluid power delivered by a pump is governed by the system’s pressure-flow relationship.The fluid power output of a pump is proportional to the head multiplied by the flow at the operating point of the system. On a pump system curve, the fluid power delivered into a system by a pump (H×Q) can be ‘seen’ as the area of the rectangle formed by drawing a horizontal line from the pump operating point onto the pressure axis and a vertical one onto the flow axis (blue square on Figure 1). The area of this rectangle and therefore the delivered fluid power, changes with every change of the pump’s operating point.
A pump’s operating point or duty point depends on the real pressures and flows that the pump ‘sees’ due to the friction head generated in the piping network and the static head pressures associated with the whole system.
Figure 2 shows two system curves. The steep system curve, arises when there is almost no static head (height difference) to overcome and almost all of the pressure experienced by the pump will be due to flow dependant friction losses. The flat system curve will arise when the friction losses are low (due to short piping distances or large pipe diameters, for example) and the fluid being lifted through a large vertical height (such as mine dewatering from the bottom of a deep mine shaft) or when the fluid is being pumped into a vessel with a high static pressure head (such as a header tank or pressurised boiler, for example.)
The all friction system with no static head is shown in Figure 3, superimposed on a family of pump curves for speeds of 70%, 80%, 90% and 100%. The operating point of the pump always falls at the intersection point of the system curve and the relevant speed-related pump curve.
In this friction-only scenario, as the speed is reduced down from 100%, the operating point moves down along the system curve. But because the static head is low, the system curve remains on or near the constant efficiency lines of the variable speed pump curve. So the pump efficiency does not change as the VSD reduces the pump speed to achieve the required flow.
More significantly, the power absorbed by the pump reduces according to the affinity laws and substantial energy savings can be achieved. It is for these systems that we talk about the cubed law of power savings.
Figure 4 shows the scenario for pumping into a system with a flat friction curve – high static head and relatively low friction losses. Again, the pump must always operate where the pump and system curves intersect but in systems with mostly static head, the system curve does not follow the affinity laws and does not follow the constant efficiency curves of the pump. This means that as the speed changes, the head will not drop off significantly while the pump efficiency will reduce. The affinity laws cannot be used to calculate the energy reduction, and savings from using a VSD will be significantly reduced.
In addition, below the 70% speed curve, the pump is in danger of being dead-headed - when the pump output pressure is less than the system head pressure, flow will drop to zero, useful pump energy (H×Q(0)) will drop to zero and all of the energy being consumed by the spinning impeller is being wasted.
Specific energy and the effect of VSDs
A useful index for comparing pumps in the same application involves calculating the energy (kWh) required to pump a specific volume of water, such as 1.0 Mℓ. This index, called Specific Energy (Es), provides the basis for comparing energy savings between different pumping systems with different control strategies. It can also be extended to enable the specific costs of pumping in these scenarios to be compared in R/Mℓ.
If we apply the specific energy calculation to the all-friction pump system shown in Figure 3, where the flow rate and pump speed reduced by 50% and the power drops by a factor of 8, we see that power drops from 79.5 kW to 10.4 kW, while the pump efficiency remains constant. These values are shown in Rows 1 and 2 of Table 1, along with the specific energy calculation, which reduce by a factor of nearly 4 for this scenario (Specific energy Es = 25% of the original). What this means is the actual energy savings are around 75%.
For a mixed static plus frictional head system scenario, in order to drop the flow by 50% from 800 m3/h to 400 m3/h the pump speed only drops by 21.5% and motor power drops from 79.5 to 34.6 kW. This looks like a substantial power saving until we look at the specific energy which has dropped from 99 to 87, an actual saving of only 12%. This is sure to disappoint pump operators expecting savings in line with the affinity laws.
And the situation with constant static head is even more dramatic. In this system, the speed only has to drop by about 13% for the flow to drop by half. Once again the power reduction of 43% sounds impressive but when we look at the specific energy figures in red we see that it has actually increased. In this scenario, adding a VSD has actually made our system less efficient by causing the pump to use more energy per unit of flow at the lower speed.
In addition, if the speed is dropped any further, it could force the pump to operate at the closed valve head (known as pump dead heading), which is a very dangerous area in terms of the life of the pump. If a pump is operated at closed valve head for anything more than a few minutes at a time, the resultant loading on the bearings and the increased temperature of the fluid can quickly lead to bearings seizing, mechanical seals failing, shafts snapping and even the possibility of the pump casing exploding.
Conclusion
So before commissioning the installation of a VSD on an existing pumping system, be sure first to determine the system curve and its static and friction head components. Then, by superimposing the system curve onto the family of pump curves at different speeds, have a look at how the system will respond to varying the speed.
To complete the picture, calculate the motor speed, power draw and specific energy for several different pump speeds. Only then can you be 100% sure that the VSD will deliver the benefits you expect.
